1. Field of the Invention
This invention relates to a method for leaching cores made of either Y.sub.2 O.sub.3 or Sm.sub.2 O.sub.3 from castings of advanced super-alloy materials.
2. Background of the Invention
Some gas turbine blades and vanes contain internal passages for the circulation of air therethrough. These passages are of such complexity that casting is the only feasible means of their fabrication.
The selection of the core and mold materials for use in the casting operation is dictated by two considerations. First, the core and the mold materials must be thermodynamically stable against interaction with the molten alloy components, in particular carbon in the instance of advanced superalloys like NiTaC-13. The reaction of carbon in the molten alloy with an oxide core and/or mold material could lead to the formation of CO or CO.sub.2 filled bubbles in the solidified structure, which is a very undesirable situation. The second consideration applies primarily to the selection of the core material. Because of the web-like nature of the internal cooling air passages, the core material must be dissolved away chemically without damage to the surrounding alloy structure. The core material, a ceramic, must therefore be chemically labile toward aqueous solutions which will not affect the alloy. This condition of lability requires that the core materials not contain materials such as Al.sub.2 O.sub.3, ThO.sub.2, ZrO.sub.2 or SiO.sub.2 and the like since these compounds generally require strongly acidic media, such as HF, for their rapid dissolution. The alloy would most likely dissolve at a higher rate than the core under these conditions.
It is therefore an object of this invention to provide a new and improved method for removing core material from castings of advanced superalloy materials without deleteriously affecting the finish and material of the casting.
Another object of this invention is to provide an aqueous solution of a weak acid to remove core materials from the castings of advanced superalloy materials.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.